Since its inception in the 1970's, the Clean Air Act (CAA) has served as the framework to protect people and the environment from the harmful effects of air pollution. A key component of the Clean Air Act is a requirement that the U.S. EPA significantly reduce emissions of the most potentially harmful air pollutants. The Clean Air Act refers to these pollutants as "hazardous air pollutants" (HAPs), but they are also known as air toxics.

Prior to 1990, the Clean Air Act required EPA to set standards for each individual air toxin based on its particular health risks. This approach was difficult and not very effective at reducing emissions. As a result, the 1990 Clean Air Act Amendments directed the U.S. EPA to use technology-based and performance-based approaches in order to significantly reduce air toxics emissions from major sources of air pollution. This approach would be followed by a risk-based approach to address any remaining risk (“residual risk.”)

Under the "technology-based" approach, EPA develops standards for controlling emissions of air toxics from each major industrial source ("source category.") These standards are known as "maximum achievable control technology (MACT) standards.” The MACT standards are based on emissions levels that are already being achieved by the better-controlled and lower-emitting sources in an industry. This approach ensures that each major source of toxic air pollution will be required to employ effective measures to limit its emissions.